Lubricating oil composition

ABSTRACT

A lubricating oil composition includes: a lubricant base having a kinematic viscosity at 100° C. of 1.0 to 10 mm 2 /s and % C P  of no less than 70; (A) 0.1 to 30 mass % of a poly(meth)acrylate viscosity index improver having a PSSI of no more than 5, a weight average molecular weight of 10,000 to 500,000, a ratio A/B of less than 2.4 and a ratio C/B of less than 1.4, the ratio A/B being a ratio of a viscosity increase effect A on kinematic viscosity at 100° C. to a viscosity increase effect B on HTHS viscosity at 150° C., and the ratio C/B being a ratio of a viscosity increase effect C on kinematic viscosity at 150° C. to the viscosity increase effect B on HTHS viscosity at 150° C.; and (B) 0.01 to 2.0 mass % of a friction modifier.

TECHNICAL FIELD

The present invention relates to lubricating oil compositions.

BACKGROUND ART

Conventionally, lubricating oils are used for internal combustion engines, transmissions, and other machineries for their smooth operation. Specifically, lubricating oils for internal combustion engines (engine oils) are required to have increasingly higher performance due to increasingly higher performance, increasingly higher power, and increasingly severe operation conditions, etc. of internal combustion engines. Therefore, various additives such as anti-wear agents, metallic detergents, ashless dispersants, and antioxidants are incorporated in conventional engine oils in order to satisfy the above required performance (for example, see Patent Literatures 1 to 3 below). Recently, much higher fuel efficiency has been required of lubricating oils, and application of high viscosity index base oils and various friction modifiers is being considered (for example, see Patent Literature 4 below).

CITATION LIST Patent Literature

Patent Literature 1: JP 2001-279287A

Patent Literature 2: JP 2002-129182A

Patent Literature 3: JP H08-302378A

Patent Literature 4: JP H06-306384A

SUMMARY OF INVENTION Technical Problem

However, conventional lubricating oils are not necessarily enough in terms of fuel efficiency.

Examples of commonly known techniques for improving fuel efficiency include reducing kinematic viscosity and increasing a viscosity index of a lubricating oil (a multigrade oil by combination of a low viscosity base oil and a viscosity index improver), and incorporating a friction reducing agent. When viscosity is decreased, lubricating performance under severe lubricating conditions (under high temperature and high shear conditions) deteriorates, which may lead to troubles such as wear, seizure, and fatigue failure, and increased evaporation loss, due to decrease of the viscosity of the lubricating oil, or base oil that constitutes the lubricating oil. Ashless or molybdenum-based friction modifiers are known as friction reducing agents. However, a fuel efficient lubricant oil which outperforms such common lubricating oil containing a friction reducing agent is demanded.

It is necessary to make HTHS viscosity at 150° C. high (“HTHS viscosity” is also called “high temperature high shear viscosity”) so as to prevent troubles due to decreased viscosity and to maintain durability. It is also necessary to make shear stability high so as to prevent viscosity decrease due to shear. It is advantageous to decrease kinematic viscosity at 40° C., kinematic viscosity at 100° C., and HTHS viscosity at 100° C. while maintaining a certain level of HTHS viscosity at 150° C. for further improving fuel efficiency while maintaining other performances for practical use. However, it is too difficult for conventional lubricating oils to satisfy all these requirements.

The present invention was made in view of such an actual situation.

An object of the present invention is to provide a lubricating oil composition of improved engine friction loss reducing performance and fuel efficiency, which keeps sufficient low kinematic viscosity at 40° C. and 100° C. at the initial stage of use thereof as well as for an extended period of use thereof, while maintaining a certain level of HTHS viscosity at 150° C.

Solution to Problem

The lubricating oil composition of the present invention comprising:

a lubricant base oil having a kinematic viscosity at 100° C. of 1.0 to 10 mm²/s and % C_(P) of no less than 70;

(A) 0.1 to 30 mass % of a poly(meth)acrylate viscosity index improver having a PSSI of no more than 5, a weight average molecular weight of 10,000 to 500,000, a ratio A/B of less than 2.4 and a ratio C/B of less than 1.4, the ratio A/B being a ratio of a viscosity increase effect A on kinematic viscosity at 100° C. represented by the following formula (1) to a viscosity increase effect B on HTHS viscosity at 150° C. represented by the following formula (2), and the ratio C/B being a ratio of a viscosity increase effect C on kinematic viscosity at 150° C. represented by the following formula (3) to the viscosity increase effect B on HTHS viscosity at 150° C. represented by the following formula (2); and

(B) 0.01 to 2.0 mass % of a friction modifier,

A=X−X ₀  (1)

wherein in the formula (1), A represents the viscosity increase effect of the viscosity index improver on kinematic viscosity at 100° C.; X represents kinematic viscosity at 100° C. (unit: mm²/s) of a mixture consisting of a reference base oil and 6 mass % of the viscosity index improver, the reference base oil being a hydrocracked base oil YUBASE™ 4 manufactured by SK Lubricants Co., Ltd.; and X₀ represents kinematic viscosity at 100° C. (unit: mm²/s) of the reference base oil;

B=Y−Y ₀  (2)

wherein in the formula (2), B represents the viscosity increase effect of the viscosity index improver on HTHS viscosity at 150° C.; Y represents HTHS viscosity at 150° C. (unit: mPa·s) of the mixture consisting of the reference base oil and 6 mass % of the viscosity index improver; and Y₀ represents HTHS viscosity at 150° C. (unit: mPa·s) of the reference base oil; and

C=Z−Z ₀  (3)

wherein in the formula (3), C represents the viscosity increase effect of the viscosity index improver on kinematic viscosity at 150° C.; Z represents kinematic viscosity at 150° C. (unit: mm²/s) of the mixture consisting of the reference base oil and 6 mass % of the viscosity index improver; and Z₀ represents kinematic viscosity at 150° C. (unit: mm²/s) of the reference base oil.

In the first embodiment, the lubricant base oil has a NOACK evaporation loss at 250° C. of less than 15 mass %.

In the second embodiment, the lubricant base oil has a NOACK evaporation loss at 250° C. of no less than 15 mass %; and the poly(meth)acrylate viscosity index improver has a weight average molecular weight of 10,000 to 400,000.

In the present application, “kinematic viscosity at 100° C.” means kinematic viscosity at 100° C., specified in ASTM D-445, and “kinematic viscosity at 150° C.” means kinematic viscosity at 150° C., specified in ASTM D-445. In addition, “% C_(P)” means percentage of the number of paraffinic carbons to the number of the total carbons which is obtained by the method conforming to ASTM D 3238-85 (ring analysis by n-d-M method). “HTHS viscosity at 150° C.” means high temperature high shear viscosity at 150° C., specified in ASTM D4683, and “HTHS viscosity at 100° C.” means high temperature high shear viscosity at 100° C., specified in ASTM D4683. The NOACK evaporation loss is evaporation loss of the lubricating oil measured conforming to ASTM D 5800. “(Meth)acrylate” means acrylate and/or methacrylate, and “poly(meth)acrylate” means a polymer including an acrylate monomer unit and/or a methacrylate monomer unit.

The ratio A/B being a ratio of a viscosity increase effect A on kinematic viscosity at 100° C. represented by the following formula (1) to a viscosity increase effect B on HTHS viscosity at 150° C. represented by the following formula (2) is one index representing fuel efficiency. When HTHS viscosity at 150° C. is tried to be maintained, a viscosity index improver having a high value of the ratio A/B may lead to insufficient fuel efficiency because the viscosity-temperature characteristics deteriorate.

The ratio C/B being a ratio of a viscosity increase effect C on kinematic viscosity at 150° C. represented by the following formula (3) to the viscosity increase effect B on HTHS viscosity at 150° C. represented by the following formula (2) is one index representing fuel efficiency. When HTHS viscosity at 150° C. is tried to be maintained, a viscosity index improver having a high value of the ratio C/B may lead to insufficient fuel efficiency because the viscosity-temperature characteristics deteriorate.

Combining the above specified lubricant base oil and the viscosity index improver having the ratio A/B of less than 2.4 and the ratio C/B of less than 1.4 makes it possible to obtain the lubricating oil composition of improved fuel efficiency while maintaining HTHS viscosity at 150° C.

Preferably, the (A) viscosity index improver has a ratio (Mw/PSSI) of the weight average molecular weight (Mw) to the PSSI of no less than 1×10⁴.

Preferably, the (B) friction modifier is a molybdenum-based friction modifier.

In the first embodiment, preferably, the lubricating oil composition has a NOACK evaporation loss at 250° C. of no more than 15 mass %.

“PSSI” in the present application means Permanent Shear Stability Index of a polymer calculated based on data measured according to ASTM D 6278-02 (Test Method for Shear Stability of Polymer Containing Fluids Using a European Diesel Injector Apparatus), conforming to ASTM D 6022-01 (Standard Practice for Calculation of Permanent Shear Stability Index).

The NOACK evaporation loss is evaporation loss of the lubricating oil measured conforming to ASTM D 5800.

In the second embodiment, the lubricating oil composition has a NOACK evaporation loss at 250° C. of no less than 15 mass %, more preferably no less than 20 mass %, and further preferably no less than 25 mass %.

In the second embodiment, preferably, the lubricant base oil is a wax isomerized base oil having a kinematic viscosity at 100° C. of 2.0 to 4.5 mm²/s, % C_(P) of no less than 85, and a NOACK evaporation loss at 250° C. of no less than 15 mass %.

Advantageous Effects of Invention

The present invention can provide a lubricating oil composition of improved durability and fuel efficiency, which can keep sufficiently low kinematic viscosity at 40° C. and 100° C. at the initial stage of use thereof as well as for an extended period of use thereof, while maintaining HTHS viscosity at 150° C., and can sufficiently suppress viscosity decrease after being sheared.

DESCRIPTION OF EMBODIMENTS

The present invention will be described hereinafter. Expression “A to B” concerning numeral values A and B means “no less than A and no more than B” unless otherwise specified. In such expression, if a unit is added only to the numeral value B, the unit is applied to the numeral value A as well. A word “or” means a logical sum unless otherwise specified.

<Lubricant Base Oil>

In the lubricating oil composition according to the first embodiment of the present invention, a lubricant base oil having a kinematic viscosity at 100° C. of 1.0 to 10 mm²/s, % C_(P) of no less than 70, and a NOACK evaporation loss at 250° C. of less than 15 mass % in (hereinafter referred to as “lubricant base oil according to the first embodiment”) is used as its base oil.

In the lubricating oil composition according to the second embodiment of the present invention, a lubricant base oil having a kinematic viscosity at 100° C. of 1.0 to 10 mm²/s, % C_(P) of no less than 70, and a NOACK evaporation loss at 250° C. of no less than 15 mass % (hereinafter referred to as “lubricant base oil according to the second embodiment”) is used as its base oil.

Examples of the lubricant base oil according to each of the first and second embodiments include paraffinic mineral oil, normal-paraffinic base oil, isoparaffinic base oil, and mixtures thereof, having kinematic viscosity at 100° C. of 1 to 10 mm²/s, and in the first embodiment a NOACK evaporation loss at 250° C. of less than 15 mass %, and in the second embodiment a NOACK evaporation loss at 250° C. of no less than 15 mass %. The above examples of oils are obtained by refining lubricant oil fractions that are obtained by distillation under atmospheric pressure and/or distillation reduced pressure of crude oil, through a refining process including solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, catalytic dewaxing, hydrorefining, sulfuric acid washing, or white clay treatment, etc., or combination thereof.

Preferred examples of the lubricant base oil according to each of the first and second embodiments include base oil obtained by (i) refining a raw material base oil of any one of the following (1) to (8) and/or lubricant oil fractions recovered from the raw material base oil, by a predetermined refining methods, and then (ii) recovering lubricant oil fractions:

(1) a distillate obtained by atmospheric distillation of paraffin-base crude oil and/or mixed base crude oil;

(2) a distillate obtained by vacuum distillation of residue of atmospheric distillation of paraffin-base crude oil and/or mixed base crude oil (WVGO);

(3) wax obtained through a lubricant oil dewaxing step (slack wax etc.) and/or synthetic wax obtained through a gas-to-liquid (GTL) process or the like (Fischer-Tropsch wax, GTL wax, etc.);

(4) mixed oil of at least one selected from the base oils (1) to (3), and/or a mild hydrocracked oil of the mixed oil;

(5) mixed oil of at least two selected from the base oils (1) to (4);

(6) deasphalted oil of the base oil (1), (2), (3), (4) or (5) (DAO);

(7) mild hydrocracked oil of the base oil (6) (MHC); and

(8) mixed oil of at least two selected from the base oils (1) to (7).

Preferable examples of the above described predetermined refining method include: hydrorefining such as hydrocracking and hydrofinishing; solvent refining such as furfural solvent extraction; dewaxing such as solvent dewaxing and catalytic dewaxing; white clay treatment using acid white clay, activated white clay, etc.; chemical (acid or alkali) washing such as sulfuric acid washing and caustic soda washing, or the like. In the present invention, these refining methods may be carried out individually, or at least two refining methods may be carried out in combination. When at least two refining methods are combined, the order thereof is not restricted, and can be suitably selected.

The following base oil (9) or (10) is especially preferable as the lubricant base oil according to each of the first and second embodiments. The base oils (9) and (10) are obtained by carrying out a predetermined process on a base oil selected from the base oils (1) to (8), or on lubricant oil fractions recovered from the selected base oil:

(9) hydrocracked base oil obtained by: hydrocracking a base oil selected from the base oils (1) to (8), or lubricant oil fractions recovered from the selected base oil; dewaxing the hydrocracked product or lubricant oil fractions recovered therefrom by distillation or the like, through solvent dewaxing, catalytic dewaxing, or the like; and optionally further distilling the dewaxed product; and

(10) hydroisomerized base oil obtained by: hydroisomerizing a base oil selected from the base oils (1) to (8), or lubricant oil fractions recovered from the selected base oil; carrying out a dewaxing process such as solvent dewaxing and catalytic dewaxing on the hydroisomerized product or lubricant oil fractions recovered therefrom by distillation or the like; and optionally further distilling the dewaxed product. Catalytic dewaxing is preferable as the dewaxing process.

When obtaining the above described lubricant base oil (9) or (10), a solvent refining process and/or hydrofinishing process may be further performed at a proper stage if necessary.

Although a catalyst used for the above described hydrocracking or hydroisomerization is not restricted, preferably employed is a hydrocracking catalyst including at least one metal having hydrogenating ability (such as at least one metal of group VIa and group VIII of the periodic table) supported on a catalyst support, the catalyst support including at least one composite oxide having cracking activity (for example, silica-alumina, alumina-boria and silica-zirconia) and the catalyst support optionally further including a binder binding the at least one composite oxide; or a hydroisomerization catalyst including at least one metal having hydrogenation ability including at least one group VIII metal, the at least one metal being supported on a catalyst support, the catalyst support including a zeolite (such as ZSM-5, zeolite beta, and SAPO-11). A hydrocracking catalyst and a hydroisomerization catalyst may be used in combination by stacking, mixing, or the like.

Reaction conditions upon hydrocracking or hydroisomerization are not restricted. Preferably, the hydrogen partial pressure is 0.1 to 20 MPa, the average reaction temperature is 150 to 450° C., LHSV is 0.1 to 3.0 hr⁻¹, and the hydrogen/oil ratio is 50 to 20000 scf/b.

The kinematic viscosity of the lubricant base oil according to each of the first and second embodiments at 100° C. is 1.0 to 10 mm²/s, preferably no more than 5 mm²/s, more preferably no more than 4.5 mm²/s, further preferably no more than 4.4 mm²/s, and especially preferably no more than 4.3 mm²/s; and preferably no less than 3.5 mm²/s, more preferably no less than 3.7 mm²/s, further preferably no less than 3.9 mm²/s, and especially preferably no less than 4.0 mm²/s. The lubricant base oil having a kinematic viscosity at 100° C. more than 10 mm²/s may lead to deteriorated low-temperature viscosity properties, and insufficient fuel efficiency; and the kinematic viscosity at 100° C. less than 1 mm²/s may lead to inferior lubricity due to insufficient oil film formation at positions to be lubricated, and increased evaporation loss.

The kinematic viscosity of the lubricant base oil according to each of the first and second embodiments at 40° C. is preferably no more than 40 mm²/s, more preferably no more than 30 mm²/s, further preferably no more than 25 mm²/s, especially preferably no more than 22 mm²/s, and most preferably no more than 20 mm²/s; and preferably no less than 6.0 mm²/s, more preferably no less than 8.0 mm²/s, further preferably no less than 10 mm²/s, especially preferably no less than 12 mm²/s, and most preferably no less than 14 mm²/s. The lubricant base oil having a kinematic viscosity at 40° C. more than 40 mm²/s may lead to deteriorated low-temperature viscosity properties, and insufficient fuel efficiency; and the kinematic viscosity at 40° C. less than 6.0 mm²/s may lead to inferior lubricity due to insufficient oil film formation at positions to be lubricated, and increased evaporation loss.

The viscosity index of the lubricant base oil according to each of the first and second embodiments is preferably no less than 100; in the first embodiment more preferably no less than 110, further preferably no less than 120, especially preferably no less than 125, and most preferably no less than 130; and in the second embodiment more preferably no less than 110, further preferably no less than 115, especially preferably no less than 120, and most preferably no less than 122. The viscosity index less than 100 may lead to deteriorated viscosity-temperature characteristics, thermal and oxidation stability, increased evaporation loss, increased friction coefficients, and deteriorated anti-wear performance.

The viscosity index in this application means a viscosity index measured conforming to JIS K 2283-1993.

The density of the lubricant base oil according to each of the first and second embodiments at 15° C. (ρ₁₅) is preferably no more than 0.860, more preferably no more than 0.850, further preferably no more than 0.840, and especially preferably no more than 0.835.

The density at 15° C. in this application means density measured at 15° C., conforming to JIS K 2249-1995.

The pour point of the lubricant base oil according to each of the first and second embodiments is preferably no more than −10° C., more preferably no more than −12.5° C., further preferably no more than −15° C., and especially preferably no more than −17° C. The pour point beyond the upper limit tends to deteriorate low-temperature fluidity of the entire lubricating oil composition. The pour point in this application means a pour point measured conforming to JIS K 2269-1987.

The sulfur content in the lubricant base oil according to each of the first and second embodiments depends on the sulfur content in its raw material. For example, in a case where a raw material that is substantially sulfur free, such as a synthetic wax component obtained through Fischer-Tropsch reaction or the like, is used, the lubricant base oil that is substantially sulfur free can be obtained. In a case where a raw material containing sulfur, such as slack wax obtained through the process of refining the lubricant base oil, and microwax obtained through a wax refining process, is used, the sulfur content in the obtained lubricant base oil is usually no less than 100 mass ppm. In the lubricant base oil according to each of the first and second embodiments, in view of improvement of thermal and oxidation stability and decreasing sulfur content, the sulfur content is preferably no more than 100 mass ppm, more preferably no more than 50 mass ppm, further preferably no more than 10 mass ppm, and most preferably no more than 5 mass ppm.

The nitrogen content in the lubricant base oil according to each of the first and second embodiments is preferably no more than 7 mass ppm, more preferably no more than 5 mass ppm, and further preferably no more than 3 mass ppm. The nitrogen content beyond 7 mass ppm tends to lead to deteriorated thermal and oxidation stability. The nitrogen content in this specification means the nitrogen content measured conforming to JIS K 2609-1990.

% C_(P) of the lubricant base oil according to each of the first and second embodiments is no less than 70, preferably no less than 80, and more preferably no less than 85; and in the second embodiment further preferably no less than 87, and especially preferably no less than 90; and usually no more than 99, preferably no more than 95, and further preferably no more than 94. The lubricant base oil having % C_(p) under the lower limit tends to lead to deteriorated viscosity-temperature characteristics, thermal and oxidation stability and friction properties, and further lead to decreased effect of additives when additives are incorporated to the lubricant base oil. The lubricant base oil having % C_(P) beyond the upper limit tends to lead to decreased solubility of additives.

Preferably, % C_(A) of the lubricant base oil according to each of the first and second embodiments is no more than 2, more preferably no more than 1, further preferably no more than 0.8, and especially preferably no more than 0.5. The lubricant base oil having % C_(A) beyond the upper limit tends to lead to deteriorated viscosity-temperature characteristics, thermal and oxidation stability and fuel efficiency.

% C_(N) of the lubricant base oil according to each of the first and second embodiments is no more than 30, and preferably 4 to 25; and in the second embodiment more preferably 5 to 13. The lubricant base oil having % C_(N) beyond the upper limit tends to lead to deteriorate viscosity-temperature characteristics, thermal and oxidation stability and friction properties. The lubricant base oil having % C_(N) under the lower limit tends to lead to decreased solubility of additives.

In this application, % C_(P), % C_(N) and % C_(A) mean percentage of the paraffinic carbons to the total carbons, percentage of the naphthenic carbons to the total carbons, and percentage of the aromatic carbons to the total carbons, respectively, obtained by the method conforming to ASTM D 3238-85 (ring analysis by n-d-M method). That is, the above described preferred ranges of % C_(P), % C_(N), and % C_(A) are based on values obtained according to the above method. For example, the value of % C_(N) obtained according to the above method can be more than 0 even if the lubricant base oil does not include the naphthene content.

The saturated content in the lubricant base oil according to each of the first and second embodiments is preferably no less than 90 mass %, preferably no less than 95 mass %, and more preferably no less than 99 mass %, on the basis of the total mass of the lubricant base oil. The cyclic-saturated content in the saturated content is preferably no more than 40 mass %, preferably no more than 35 mass %, preferably no more than 30 mass %, more preferably no more than 25 mass %, and further preferably no more than 21 mass %; and preferably no less than 5 mass %, and more preferably no less than 10 mass %. The saturated content, and the cyclic-saturated content in the saturated content within the above range makes it possible to improve the viscosity-temperature characteristics, and thermal and oxidation stability. In a case where an additive is incorporated into the lubricant base oil, it is possible to make the additive function at higher levels while keeping the additive sufficiently stably dissolved in the lubricant base oil. It further makes it possible to improve friction properties of the lubricant base oil itself, and thus improve friction reducing performance and thus energy saving performance.

In this application, the saturated content represents a value measured conforming to ASTM D 2007-93.

Any similar method according to which the same result is obtained can be used for a separation method for the saturated content, or composition analysis of the cyclic saturated content, the acyclic saturated content, and the like. Examples thereof include the method specified in the above ASTM D 2007-93, the method specified in ASTM D 2425-93, the method specified in ASTM D 2549-91, methods using high performance liquid chromatography (HPLC), and improved methods of them.

The aromatic content in the lubricant base oil according to each of the first and second embodiments is, on the basis of the total mass of the lubricant base oil, preferably no more than 5 mass %, more preferably no more than 4 mass %, further preferably no more than 3 mass %, and especially preferably no more than 2 mass %; and is preferably no less than 0.1 mass %, more preferably no less than 0.5 mass %, further preferably no less than 1 mass %, and especially preferably no less than 1.5 mass %. The aromatic content beyond the upper limit tends to lead to deteriorated viscosity-temperature characteristics, thermal and oxidation stability, and friction properties, and further to increased evaporation loss, deteriorated low-temperature viscosity properties, and furthermore, tends to lead to deteriorated effect of an additive in a case where the additive is incorporated into the lubricant base oil. The lubricant base oil according to each of the first and second embodiments may contain no aromatic content. However, the lower limit or above of the aromatic content makes it possible to further improve solubility of additives.

In this application, the aromatic content represents a value measured conforming to ASTM D 2007-93. The aromatic content usually includes alkylbenzenes, and alkylnaphthalenes; anthracenes, phenanthrenes and alkylated compounds thereof; compounds having four or more fused benzene rings; and aromatic compounds having a heteroatom such as pyridines, quinolines, phenols, and naphthols.

Synthetic base oil may be used as the lubricant base oil according to each of the first and second embodiments. Examples of synthetic base oil include poly α-olefin and hydrogenated product thereof, isobutene oligomer and hydrogenated product thereof, isoparaffin, alkylbenzene, alkylnaphthalene, diestes (such as ditridecyl glutarate, di-2-ethylhexyl azipate, diisodecyl azipate, ditridecyl azipate, and di-2-ethylhexyl sebacate), polyol esters (such as trimethylolpropane caprylate, trimethylolpropane pelargonate, pentaerythritol 2-ethylhexanoate, and pentaerythritol pelargonate), polyoxyalkylene glycol, dialkyl diphenyl ether, polyphenyl ether, and mixtures thereof; each having a kinematic viscosity at 100° C. of 1 to 10 mm²/s, and in the first embodiment a NOACK evaporation loss at 250° C. of less than 15 mass %, and in the second embodiment a NOACK evaporation loss at 250° C. of no less than 15 mass %. Among them, poly α-olefins are preferable. Examples of poly α-olefins typically include oligomers and co-oligomers of α-olefins having a carbon number of 2-32, preferably 6-16 (such as 1-octene oligomers, decene oligomers, and ethylene-propylene co-oligomers), and hydrogenated products thereof.

A method for producing a poly α-olefin is not restricted. Examples thereof include polymerizing an α-olefin in the presence of a polymerization catalyst such as a Friedel-Crafts catalyst, containing a complex of aluminum trichloride or boron trifluoride, and water, an alcohol (such as ethanol, propanol, and butanol), a carboxylic acid or an ester.

The lubricant base oil according to the second embodiment is preferably wax isomerized base oil among the above various examples of base oils. Wax isomerized base oil widely encompasses isomerized product of wax such as petroleum wax. In the present invention, any wax isomerized base oil can be used as long as satisfying the following properties. Wax isomerized base oil may be a mixture of at least two wax isomerized base oils. In the case of a mixture, the mixture has only to satisfy the following properties.

The kinematic viscosity of wax isomerized base oil at 100° C. is preferably no less than 2.0 mm²/s, more preferably no less than 2.3 mm²/s, and further preferably no less than 2.5 mm²/s. The kinematic viscosity at 100° C. of 2.0 mm²/s or more makes it possible to have sufficient lubricity, and to suppress evaporation loss of the lubricant base oil.

On the other hand, the kinematic viscosity at 100° C. is preferably no more than 4.5 mm²/s, more preferably no more than 4.2 mm²/s, and further preferably no more than 4.0 mm²/S. The kinematic viscosity at 100° C. of 4.5 mm²/s or less makes it possible to obtain the lubricating oil composition having good low-temperature viscosity properties.

Wax isomerized base oil is preferably combination of at least one selected from light base oils having a kinematic viscosity at 100° C. of no less than 2.0 mm²/s and less than 3.3 mm²/s and a viscosity index of no less than 110, and at least one selected from medium viscosity base oils having a kinematic viscosity at 100° C. of no less than 3.3 mm²/s and less than 4.5 mm²/s and a viscosity index of no less than 125.

For example, each of the above described light base oil and medium viscosity base oil may be either an individual base oil, or a mixture of two or more base oils.

The viscosity index of wax isomerized base oil is preferably no less than 110, more preferably no less than 120, and further preferably no less than 130. The viscosity index of 110 or more makes it possible to obtain a composition which shows good viscosity characteristics from low temperature to high temperature.

Although the upper end is not specifically limited, the upper limit is usually no more than 200, and preferably no more than 160. A too high viscosity index tends to lead to higher viscosity at low temperature.

The pour point of wax isomerized base oil is preferably no more than −10° C., more preferably no more than −12.5° C., further preferably no more than −15° C., and especially preferably no more than −20.0° C. Although the lower end is not specifically limited, the lower limit is preferably no less than −50° C., more preferably no less than −45° C., further preferably no less than −40° C., and most preferably no less than −37.5° C. in view of decrease of a viscosity index due to too low lower limit, and economic efficiency in a dewaxing process. The pour point of wax isomerized base oil of −10° C. or less makes it possible to obtain the lubricating oil composition having good low temperature viscosity properties. The pour point thereof less than −50° C. leads to an insufficient viscosity index.

Preferably, % C_(P) of wax isomerized base oil is no less than 80, more preferably no less than 85, and especially preferably no less than 90 in view of possible improvement of the thermal and oxidation stability, and viscosity-temperature characteristics.

Preferably, % C_(N) of wax isomerized base oil is no more than 20, more preferably no more than 15, and further preferably no more than 12; preferably no less than 2; and in view of further improving a metal fatigue lifetime, more preferably no less than 3, further preferably no less than 5, and especially preferably no less than 7.

Preferably, % C_(A) of wax isomerized base oil is no more than 1, more preferably no more than 0.5, further preferably no more than 0.1, and most preferably 0. If % C_(A) is beyond 1, the thermal and oxidation stability deteriorates.

A method for producing wax isomerized base oil is not restricted as long as the wax isomerized base oil has the above properties. Specific examples include hydroisomerized mineral oil obtained by: hydroisomerizing a wax selected from the following (i) to (iii); dewaxing, through solvent dewaxing, catalytic dewaxing, or the like, the hydroisomerized product or lubricant oil fractions recovered therefrom by distillation or the like; and optionally further distilling the dewaxed product.

Either solvent dewaxing or catalytic dewaxing may be employed as a dewaxing process for producing wax isomerized base oil. A catalytic dewaxing process is especially preferable in view of further improvement of low-temperature viscosity properties.

(i) wax obtained through a lubricant oil dewaxing step (slack wax etc.) and/or synthetic wax obtained through a gas-to-liquid (GTL) process or the like (Fischer-Tropsch wax, GTL wax, etc.);

(ii) mild hydrocracked oil of one selected from (i) or of mixed wax of at least two selected from (i); and

(iii) mixed oil obtained by at least two selected from the above described (i) and (ii).

NOACK evaporation loss of the lubricant base oil at 250° C. according to the second embodiment is no less than 15 mass %, preferably no less than 20 mass %, and more preferably no less than 25 mass %. The upper limit of the NOACK evaporation loss of the lubricant base oil at 250° C. according to the second embodiment is not restricted, but is typically no more than 50 mass %, and preferably no more than 40 mass %.

As regards the lubricant base oil according to the first embodiment, as far as the total base oil has a kinematic viscosity at 100° C. of 1.0 to 10 mm²/s, % C_(P) of 70 or more, and a NOACK evaporation loss at 250° C. of less than 15 mass %, it may be: a single base oil component having a kinematic viscosity at 100° C. of 1 to 10 mm²/s, % C_(P) of 70 or more, and a NOACK evaporation loss at 250° C. of 15 mass % or less may be individually used; a mixture of at least two base oils each having a kinematic viscosity at 100° C. of 1 to 10 mm²/s, % C_(P) of 70 or more, and a NOACK evaporation loss at 250° C. of less than 15 mass %; or a mixture of at least one first base oil having a kinematic viscosity at 100° C. of 1 to 10 mm²/s, % C_(P) of 70 or more, and a NOACK evaporation loss at 250° C. of less than 15 mass %, and at least one second base oil not having a kinematic viscosity at 100° C. of 1 to 10 mm²/s or having % C_(P) of less than 70 or having a NOACK evaporation loss at 250° C. of no less than 15 mass %.

The second base oil (base oil not having a kinematic viscosity at 100° C. of 1.0 to 10 mm²/s or having % C_(P) of less than 70 or having a NOACK evaporation loss at 250° C. of no less than 15 mass %) used in combination with the first base oil having a kinematic viscosity at 100° C. of 1.0 to 10 mm²/s, % C_(P) of 70 or more, and a NOACK evaporation loss at 250° C. of less than 15 mass % is not restricted, but examples of the second base oil include mineral base oils such as solvent refined mineral oil, hydrocracked mineral oil, hydrorefined mineral oil, and solvent dewaxed base oil, each having a kinematic viscosity at 100° C. of more than 10 mm²/s and no more than 1000 mm²/s, and synthetic base oils such as the above described synthetic base oils each having a kinematic viscosity at 100° C. outside the range of 1.0 to 10 mm²/s.

As regards the lubricant base oil according to the second embodiment, as far as the total base oil has a kinematic viscosity at 100° C. of 1.0 to 10 mm²/s, % C_(P) of 70 or more, and a NOACK evaporation loss at 250° C. of no less than 15 mass %, it may be: a single base oil component having a kinematic viscosity at 100° C. of 1 to 10 mm²/s, % C_(P) of 70 or more, and a NOACK evaporation loss at 250° C. of 15 mass % or more; a mixture of at least two base oils each having a kinematic viscosity at 100° C. of 1 to 10 mm²/s, % C_(P) of 70 or more, and a NOACK evaporation loss at 250° C. of 15 mass % or more; or a mixture of at least one third base oil having a kinematic viscosity at 100° C. of 1 to 10 mm²/s, % C_(P) of 70 or more, and a NOACK evaporation loss at 250° C. of 15 mass % or more, and at least one fourth base oil not having a kinematic viscosity at 100° C. of 1.0 to 10 mm²/s or having % C_(P) of less than 70 or having a NOACK evaporation loss at 250° C. of less than 15 mass %.

The fourth base oil (base oil not having kinematic viscosity at 100° C. of 1.0 to 10 mm²/s or having % C_(P) of less than 70 or having NOACK evaporation loss at 250° C. of less than 15 mass %) used in combination with the third base oil having a kinematic viscosity at 100° C. of 1.0 to 10 mm²/s, % C_(P) of 70 or more, and a NOACK evaporation loss at 250° C. of 15 mass % or more is not restricted, but examples of the fourth base oil include mineral base oils such as solvent refined mineral oil, hydrocracked mineral oil, hydrorefined mineral oil, and solvent dewaxed base oil, each having a kinematic viscosity at 100° C. more than 10 mm²/s and no more than 1000 mm²/s, and synthetic base oils such as the above described synthetic base oils each having a kinematic viscosity at 100° C. outside the range of 1.0 to 10 mm²/s.

In a case where the lubricant base oil according to the second embodiment is a mixed base oil of the third base oil having a kinematic viscosity at 100° C. of 1.0 to 10 mm²/s, % C_(P) of 70 or more, and a NOACK evaporation loss at 250° C. of 15 mass % or more, and the fourth base oil not having kinematic viscosity at 100° C. of 1.0 to 10 mm²/s or having % C_(P) of below 70 or having NOACK evaporation loss at 250° C. of less than 15 mass %, the content of the third base oil in the mixed base oil is preferably no less than 30 mass %, more preferably no less than 50 mass %, and further preferably no less than 70 mass %.

The content of the lubricant base oil of the first embodiment in the lubricating oil composition of the first embodiment, and the content of the lubricant base oil of the second embodiment in the lubricating oil composition of the second embodiment, are usually no less than 75 mass %, preferably no less than 85 mass %, and usually no more than 95 mass %, on the basis of the total mass of the lubricating oil composition, respectively.

<(A) Poly(Meth)Acrylate Viscosity Index Improver>

The lubricating oil composition according to the first embodiment of the present invention contains (A) 0.1 to 30 mass % of a poly(meth)acrylate viscosity index improver having a PSSI of no more than 5, a molecular weight of 10,000 to 500,000, a ratio A/B of less than 2.4 and a ratio C/B of less than 1.4, the ratio A/B being a ratio of a viscosity increase effect A on kinematic viscosity at 100° C. represented by the following formula (1) to a viscosity increase effect B on HTHS viscosity at 150° C. represented by the following formula (2), and the ratio C/B being a ratio of a viscosity increase effect C on kinematic viscosity at 150° C. represented by the following formula (3) to the viscosity increase effect B on HTHS viscosity at 150° C. represented by the following formula (2) (hereinafter referred to as “viscosity index improver according to the first embodiment”, for convenience). It is possible for the lubricating oil composition according to the first embodiment of the present invention to improve fuel efficiency by containing the viscosity index improver according to the first embodiment.

The lubricating oil composition according to the second embodiment of the present invention contains (A) 0.1 to 30 mass % of a poly(meth)acrylate viscosity index improver having a PSSI of no more than 5, a molecular weight of 10,000 to 400,000, a ratio A/B of less than 2.4 and a ratio C/B of less than 1.4, the ratio A/B being a ratio of a viscosity increase effect A on kinematic viscosity at 100° C. represented by the following formula (1) to a viscosity increase effect B on HTHS viscosity at 150° C. represented by the following formula (2), and the ratio C/B being a ratio of a viscosity increase effect C on kinematic viscosity at 150° C. represented by the following formula (3) to the viscosity increase effect B on HTHS viscosity at 150° C. represented by the following formula (2) (hereinafter referred to as “viscosity index improver according to the second embodiment”, for convenience). It is possible for the lubricating oil composition according to the second embodiment of the present invention to improve fuel efficiency by containing the viscosity index improver according to the second embodiment.

The structure of the compound of the viscosity index improver according to each of the first and second embodiments is not restricted as far as the above requirements are satisfied. Specific examples of the compound include non-dispersant or dispersant poly(meth)acrylate viscosity index improvers, (meth)acrylate-olefin copolymers, and mixtures thereof.

A=X−X ₀  (1)

wherein in the formula (1), A represents the viscosity increase effect on kinematic viscosity at 100° C.; X represents kinematic viscosity at 100° C. (unit: mm²/s) of a mixture consisting of a reference base oil and 6 mass % of the viscosity index improver, the reference base oil being a hydrocracked base oil YUBASE™ 4 manufactured by SK Lubricants Co., Ltd.; and X₀ represents kinematic viscosity at 100° C. (unit: mm²/s) of the reference base oil;

B=Y−Y ₀  (2)

wherein in the formula (2), B represents the viscosity increase effect on HTHS viscosity at 150° C.; Y represents HTHS viscosity at 150° C. (unit: mPa·s) of the mixture consisting of the reference base oil and 6 mass % of the viscosity index improver; and Y₀ represents HTHS viscosity at 150° C. (unit: mPa·s) of the reference base oil; and

C=Z−Z ₀  (3)

wherein in the formula (3), C represents the viscosity increase effect on kinematic viscosity at 150° C.; Z represents kinematic viscosity at 150° C. (unit: mm²/s) of the mixture consisting of the reference base oil and 6 mass % of the viscosity index improver; and Z₀ represents kinematic viscosity at 150° C. (unit: mm²/s) of the reference base oil.

In the above formulae (1) to (3), “a hydrocracked base oil YUBASE™ 4 manufactured by SK Lubricants Co., Ltd.”, which is the reference base oil used for measurement of the viscosity increase effects, is commercially available mineral base oil, and properties thereof are shown in the following Table 1:

TABLE 1 YUBASE4 Density (15° C.) g/cm³ 0.835 Kinematic Viscosity (40° C.) mm²/s 20.0 (100° C.) mm²/s 4.29 Viscosity Index 123 Pour Point ° C. −17.5 NOACK Evaporation Loss (250° C., 1 h) mass % 14 Aniline Point ° C. 116 Iodine Number 0.05 S Content mass ppm <1 N Content mass ppm <3 Analysis by n-d-M Method % C_(P) 80.7 % C_(N) 19.3 % C_(A) 0 Chromatographic Analysis mass % Saturated Cont. 99.7 Aromatic Cont. 0.2 Resin Cont. 0.1 Recovery Rate 100 Paraffin Cont. on the basis of mass % 53.8 Saturated Cont. Naphthene Cont. on the basis of mass % 46.2 Saturated Cont. Producing Method Hydro- cracking

The viscosity increase effects A, B and C of the viscosity index improver can be obtained by measuring the kinematic viscosity at 100° C. X₀, X, the HTHS viscosity at 150° C. Y₀, Y, and the kinematic viscosity at 150° C. Z₀, Z before and after 6 mass % of the viscosity index improver is added to the reference base oil, and calculating the differences X−X₀, Y−Y₀, and Z−Z₀.

The ratio A/B of the viscosity increase effect of the viscosity index improver needs to be below 2.4 as described above, and is preferably no more than 2.3, and more preferably no more than 2.1. The lower limit of the ratio A/B is not restricted, but preferably no less than 1.3, and more preferably no less than 1.5.

The ratio C/B of the viscosity increase effect of the viscosity index improver needs to be below 1.4, and is preferably no more than 1.3, and further preferably no more than 1.25. The lower limit of the ratio C/B is not restricted, but preferably no less than 0.4, and more preferably no less than 0.6.

The viscosity index improver according to each of the first and second embodiments preferably contains a poly(meth)acrylate viscosity index improver comprising 10-90 mol % of the structural units represented by the following general formula (1) on the basis of the total monomer units in the polymer.

[In the general formula (1), R¹ is hydrogen or a methyl group, and R² is a linear or branched chain hydrocarbon group having a carbon number of 1 to 6.]

In the viscosity index improver according to each of the first and second embodiments, the content of the (meth)acrylate structural units represented by the general formula (1) in the polymer is preferably 10 to 90 mol %, more preferably no more than 80 mol %, and further preferably no more than 70 mol %; and more preferably no less than 20 mol %, further preferably no less than 30 mol %, and especially preferably no less than 40 mol %. The content of the (meth)acrylate structural units represented by the general formula (1) on the basis of the total monomer units of the polymer beyond 90 mol % may lead to inferior solubility in the base oil, inferior improvement effect on viscosity-temperature characteristics, and inferior low-temperature viscosity characteristics. The content under 20 mol % may lead to inferior improvement effect on viscosity-temperature characteristics.

The viscosity index improver according to each of the first and second embodiments may be a copolymer comprising another (meth)acrylate structural unit in addition to the (meth)acrylate structural unit represented by the general formula (1). Such a copolymer can be obtained by copolymerizing one or more monomer(s) represented by the following general formula (2) (hereinafter referred to as “monomer (M-1)”), and a monomer other than the monomer (M-1).

[In the general formula (2), R³ is a hydrogen atom or a methyl group, and R⁴ is a linear or branched chain hydrocarbon group having a carbon number of 1 to 6.]

Any monomer can be combined with the monomer (M-1). For example, a monomer represented by the following general formula (3) (hereinafter referred to as “monomer (M-2)”) is preferable. A copolymer of the monomer (M-1) and the monomer (M-2) is a so-called non-dispersant poly(meth)acrylate viscosity index improver.

[In the general formula (3), R⁵ is a hydrogen atom or a methyl group, and R⁶ is a linear or branched chain hydrocarbon group having a carbon number of 7 or more.]

R⁶ in the monomer (M-2) represented by the formula (3) is a linear or branched chain hydrocarbon group having a carbon number of 7 or more as described above, preferably a linear or branched chain hydrocarbon group having a carbon number of 10 or more, further preferably a linear or branched chain hydrocarbon group having a carbon number of 15 or more, and more preferably a branched chain hydrocarbon group having a carbon number of 18 or more. The upper limit of the carbon number of the hydrocarbon group represented by R⁶ is not restricted, but this hydrocarbon group is preferably a linear or branched chain hydrocarbon group having the carbon number of 50,000 or less, more preferably a linear or branched chain hydrocarbon group having the carbon number of 500 or less, further preferably a linear or branched chain hydrocarbon group having the carbon number of 100 or less, especially preferably a branched chain hydrocarbon group having the carbon number of 50 or less, and most preferably a branched chain hydrocarbon group having the carbon number of 25 or less.

One example of the viscosity index improver according to each of the first and second embodiments is comb-shaped poly(meth)acrylate. Comb-shaped poly(meth)acrylate here means a copolymer of the monomer (M-1) and the monomer (M-2), wherein the monomer (M-2) is a macromonomer including R⁶ in the formula (3) having a number average molecular weight (Mn) of 1,000 to 10,000 (preferably 1,500 to 8,500, and more preferably 2,000 to 7,000). Examples of such a macromonomer include a macromonomer derived from a hydrogenated product of a polyolefin obtained by copolymerizing butadiene and isoprene.

In the viscosity index improver according to each of the first and second embodiments, the polymer may comprise either only one kind of (meth)acrylate structural units corresponding to the monomer (M-2) represented by the general formula (3), or combination of two or more kinds thereof. The content of the structural units corresponding to the monomer (M-2) is, on the basis of the total monomer units of the polymer, preferably 0.5 to 70 mol %, more preferably no more than 60 mol %, further preferably no more than 50 mol %, especially preferably no more than 40 mol %, and most preferably no more than 30 mol %; and preferably no less than 1 mol %, more preferably no less than 3 mol %, further preferably no less than 5 mol %, and especially preferably no less than 10 mol %. The content of the structural units corresponding to the monomer (M-2) represented by the general formula (3) on the basis of the total monomer units of the polymer beyond 70 mol % may lead to inferior improvement effect on viscosity-temperature characteristics, and inferior low-temperature viscosity characteristics. The content under 0.5 mol % may lead to inferior improvement effect on viscosity-temperature characteristics.

One or more selected from a monomer represented by the following general formula (4) (hereinafter referred to as “monomer (M-3)”), and a monomer represented by the following general formula (5) (hereinafter referred to as “monomer (M-4)”) is/are preferable as other monomers to be combined with the monomer (M-1). A copolymer of the monomer (M-1) and the monomer (M-3) and/or (M-4) is a so-called dispersant poly(meth)acrylate viscosity index improver. This dispersant poly(meth)acrylate viscosity index improver may further contain the monomer (M-2) as constituting monomer.

[In the general formula (4), R⁷ is a hydrogen atom or a methyl group, R⁸ is an alkylene group having a carbon number of 1 to 18, E¹ is an amine residue or heterocyclic residue having 1 to 2 nitrogen atoms, and 0 to 2 oxygen atoms, and a is 0 or 1.]

Specific examples of an alkylene group having a carbon number of 1 to 18 represented by R⁸ include ethylene group, propylene group, butylene group, pentylene group, hexylene group, heptylene group, octylene group, nonylene group, decylene group, undecylene group, dodecylene group, tridecylene group, tetradecylene group, pentadecylene group, hexadecylene group, heptadecylene group, and octadecylene group (each alkylene group may be either a linear or branched chain).

Specific examples of a residue represented by E′ include dimethylamino group, diethylamino group, dipropylamino group, dibutylamino group, anilino group, toluidino group, quinolidino group, acetylamino group, benzoylamino group, morpholino group, pyrrolyl group, pyrrolino group, pyridyl group, methylpyridyl group, pyrrolidinyl group, piperidinyl group, quinolyl group, pyrrolidonyl group, pyrrolidono group, imidazolino group, and pyrazino group.

[In the general formula (5), R⁹ is a hydrogen atom or a hydrocarbon group, and E² is a hydrocarbon group, or an amine residue or heterocyclic residue having 1 to 2 nitrogen atoms, and 0 to 2 oxygen atoms]

Specific examples of a group represented by E² include dimethylamino group, diethylamino group, dipropylamino group, dibutylamino group, aniline group, toluidino group, quinolidino group, acetylamino group, benzoylamino group, morpholino group, pyrrolyl group, pyrrolino group, pyridyl group, methylpyridyl group, pyrrolidinyl group, piperidinyl group, quinolyl group, pirrolidonyl group, pyrrolidono group, imidazolino group, and pyrazino group.

Preferred specific examples of the monomers (M-3) and (M-4) include dimethylaminomethyl methacrylate, diethylaminomethyl methacrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, 2-methyl-5-vinylpyridine, morpholinomethyl methacrylate, morpholinoethyl methacrylate, N-vinylpyrrolidone, and mixtures thereof.

Although the copolymerization molar ratio of the copolymer of the monomer (M-1) and the monomers (M-2) to (M-4) is not restricted, monomer (M-1):monomers (M-2) to (M-4) is preferably approximately 20:80 to 90:10, more preferably 30:70 to 80:20, and further preferably 40:60 to 70:30.

The viscosity index improver according to each of the first and second embodiments may be produced by any method. For example, they can be easily obtained by radical solution polymerization of the monomer (M-1) and/or (M-2), and one or more selected from the monomers (M-3) to (M-4) under presence of a polymerization initiator such as benzoyl peroxide.

PSSI of the viscosity index improver according to each of the first and second embodiments measured by the diesel injector method is no more than 5, more preferably no more than 4, further preferably no more than 3, especially preferably no more than 2, and most preferably no more than 1. The PSSI greater than 5 means low shear stability, and may lead to deteriorated fuel efficiency at the initial stage of use so as to maintain a certain level of kinematic viscosity and HTHS viscosity after use. The lower limit of the PSSI of the viscosity index improver according to each of the first and second embodiments is not restricted, but usually greater than 0.

The weight average molecular weight (Mw) of the viscosity index improver according to the first embodiment is 10,000 to 500,000, preferably no less than 20,000, more preferably no less than 50,000, further preferably no less than 100,000, and especially preferably no less than 120,000; and preferably no more than 400,000, more preferably no more than 300,000, and further preferably no more than 200,000.

The weight average molecular weight (Mw) of the viscosity index improver according to the second embodiment is 10,000 to 400,000, preferably no less than 20,000, more preferably no less than 50,000, further preferably no less than 100,000, and especially preferably no less than 120,000; and preferably no more than 300,000, more preferably no more than 250,000, and further preferably no more than 200,000.

When the weight average molecular weight is under the lower limit, not only the viscosity index improvement is small and fuel efficiency and low-temperature viscosity characteristics deteriorate when the viscosity index improver is dissolved in the lubricant base oil, but also the cost might rise. When the weight average molecular weight is over the upper limit, not only viscosity increase effect is too large, and fuel efficiency and low-temperature viscosity characteristics deteriorate, but also shear stability, the solubility in the lubricant base oil, and storage stability deteriorate.

The ratio of the weight average molecular weight to PSSI (Mw/PSSI) of the viscosity index improver according to each of the first and second embodiments is preferably no less than 1.0×10⁴, more preferably no less than 2.0×10⁴, further preferably no less than 5.0×10⁴, and especially preferably no less than 8.0×10⁴. Mw/PSSI less than 1.0×10⁴ may lead to deteriorated fuel efficiency and low temperature startability, that is, deteriorated viscosity temperature characteristics and low-temperature viscosity characteristics.

The ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) (Mw/Mn) of the viscosity index improver according to each of the first and second embodiments is preferably no more than 4.0, more preferably no more than 3.5, further preferably no more than 3.0, especially preferably no more than 2.0, and most preferably no more than 1.5; and preferably no less than 1.0, more preferably no less than 1.05, and further preferably no less than 1.1. Mw/Mn beyond 4.0 may lead to deteriorated solubility and deteriorated improvement effect of viscosity temperature characteristics, which may lead to failure to maintain sufficient storage stability and fuel efficiency.

The content of the viscosity index improver of the first embodiment in the lubricating oil composition of the first embodiment, and the content of the viscosity index improver of the second embodiment in the lubricating oil composition of the second embodiment are, on the basis of the total mass of the composition, 0.1 to 30 mass %, preferably no less than 0.5 mass %, more preferably no less than 1 mass %, and further preferably no less than 2 mass %; preferably no more than 50 mass %, more preferably no more than 20 mass %, and further preferably no more than 15 mass %, respectively. The content less than 0.1 mass % may lead to deteriorated fuel efficiency, and insufficient low temperature characteristics. The content beyond 30 mass % may lead to deteriorated fuel efficiency of the composition and shear stability.

The lubricating oil composition of the present invention may further contain a conventional non-dispersant or dispersant poly(meth)acrylate, a non-dispersant or dispersant ethylene-α-olefin copolymer or hydrogenated product thereof, polyisobutylene or hydrogenated product thereof, a hydrogenated styrene-diene copolymer, a styrene-maleic anhydride/ester copolymer, polyalkylstyrene, etc. in addition to the above described viscosity index improver according to the first or second embodiments

<(B) Friction Modifier>

The lubricating oil composition of the present invention comprises 0.01 to 2.0 mass % of a (B) friction modifier on the basis of the total mass of the composition. Whereby, the fuel efficiency can be improved. The (B) friction modifier is preferably at least one fiction modifier selected from organic molybdenum compounds and ashless friction modifiers.

Examples of organic molybdenum compounds include sulfur-containing organic molybdenum compounds such as molybdenum dithiophosphate, molybdenum dithiocarbamate (MoDTC); complexes of molybdenum compounds (examples thereof include: molybdenum oxide such as molybdenum dioxide and molybdenum trioxide; molybdenum acids such as orthomolybdic acid, paramolybdic acid, sulfurized (poly)molybdic acid; molybdic acid salts such as metal salts and ammonium salts of these molybdic acids; molybdenum sulfides such as molybdenum disulfide, molybdenum trisulfide, molybdenum pentasulfide, molybdenum polysulfide; thiomolybdic acid; metal salts and ammonium salts of thiomolybdic acid; and molybdenum halides such as molybdenum chloride), and sulfur-containing organic compounds (examples thereof include: alkyl (thio)xanthate, thiadiazole, mercaptothiadiazole, thiocarbonate, tetrahydrocarbylthiuram disulfide, bis(di(thio)hydrocarbyldithiophosphonate) disulfide, organic (poly)sulfide, and sulfurized ester) or other organic compounds; and sulfur-containing organic molybdenum compounds such as complexes of sulfur-containing molybdenum compounds such as the above described molybdenum sulfides, sulfurized molybdic acids, and alkenylsuccinimide. A sulfur-containing organic molybdenum friction modifier is preferably used in order to improve the fuel efficiency.

An organic molybdenum compound which does not contain sulfur as constituting element can be used as an organic molybdenum compound. Specific examples of an organic molybdenum compound which does not contain sulfur as constituting element include molybdenum-amine complex, molybdenum-succinimide complex, molybdenum salt of organic acids, and molybdenum salt of alcohols. Among them, molybdenum-amine complex, molybdenum salt of organic acids, and molybdenum salt of alcohols are preferable.

When an organic molybdenum compound is used as the (B) friction modifier in the lubricating oil composition of the present invention, the content thereof is 0.01 to 2.0 mass % on the basis of the total mass of the composition. The content of an organic molybdenum compound in terms of molybdenum is, on the basis of the total mass of the lubricating oil composition, preferably no less than 0.001 mass %, more preferably no less than 0.005 mass %, further preferably no less than 0.01 mass %, and especially preferably no less than 0.03 mass %; and preferably no more than 0.2 mass %, more preferably no more than 0.1 mass %, further preferably no more than 0.08 mass %, and especially preferably no more than 0.06 mass %. The content below the lower limit tends to lead to insufficient friction reducing effect despite addition of the compound, and insufficient fuel efficiency, and thermal and oxidation stability of the lubricating oil composition. On the other hand, the content beyond the upper limit does not bring the effect corresponding thereto, and tends to lead to deteriorated storage stability of the lubricating oil composition.

Any compound usually used as an ashless friction modifier for lubricating oil can be non-limitedly used as an ashless friction modifier. Examples of an ashless friction modifier usable in the lubricating oil composition of the present invention include compounds each having one or more heteroatoms selected from oxygen, nitrogen, and sulfur, and each having a carbon number of 6-50. More specific examples thereof include ashless friction modifiers such as amine compounds, fatty acid esters, fatty acid amides, fatty acids, aliphatic alcohols, aliphatic esters, urea compounds, and hydrazide compounds, each of which has at least one alkyl or alkenyl group having a carbon number of 6-30, especially a linear alkyl group, a linear alkenyl group, a branched alkyl group, or a branched alkenyl group having a carbon number of 6-30, in the molecule.

The content of an ashless friction modifier in the lubricating oil composition of the present invention is preferably no less than 0.01 mass %, more preferably no less than 0.1 mass %, and further preferably no less than 0.3 mass %; and preferably no more than 2 mass %, and more preferably no more than 1 mass % on the basis of the total mass of the lubricating oil composition. The content of an ashless friction modifier less than 0.01 mass % tends to lead to insufficient friction reducing effect despite addition thereof. The content thereof beyond 2 mass % tends to inhibit effect of anti-wear additives or the like, or to lead to deteriorated solubility of additives.

The (B) friction modifier in the present invention is preferably an organic molybdenum friction modifier, more preferably a sulfur-containing organic molybdenum compound, and further preferably molybdenum dithiocarbamate.

<Other Additives>

Other additives that are generally used in lubricating oil can be incorporated in the lubricating oil composition of the present invention according to its purpose in order to further improve its performance. Examples of such additives include additives such as metallic detergents, ashless dispersants, anti-wear agents (or extreme-pressure agents), antioxidants, corrosion inhibitors, anti-rust agents, demulsifiers, metal deactivators, and defoaming agents.

Examples of metallic detergents include neutral salts, basic salts, and overbased salts of alkali metal or alkaline earth metal sulfonates, alkali metal or alkaline earth metal phenates, and alkali metal or alkaline earth metal salicylates. In the present invention, at least one alkali metal or alkaline earth metal detergent selected from above, especially at least one alkaline earth metal detergent can be preferably used. Especially, magnesium salts and/or calcium salts are preferable, and calcium salts are more preferably used.

Ashless dispersants used for lubricating oil can be non-limitedly used as ashless dispersants. Examples of ashless dispersants usable in the present invention include mono or bis-succinimide having at least one linear or branched chain alkyl or alkenyl group having a carbon number of 40-400, benzylamines having at least one alkyl or alkenyl group having a carbon number of 40-400, polyamines having at least one alkyl or alkenyl group having a carbon number of 40-400, and their modified product by boron compounds, carboxylic acids, phosphoric acid, etc. At least one selected among them can be incorporated.

Examples of antioxidants include ashless antioxidant such as phenol-based or amine-based ashless antioxidant, and metal-based antioxidant such as copper-based or molybdenum-based antioxidant. Specific examples of phenol-based ashless antioxidant include 4,4′-methylenebis(2,6-di-tert-butylphenol), and 4,4′-bis(2,6-di-tert-butylphenol), and those of amine-based ashless antioxidant include phenyl-α-naphthylamine, alkylphenyl-α-naphthylamine, and dialkyldiphenylamine.

Any anti-wear agents or extreme-pressure agents used for lubricating oil can be non-limitedly used as an anti-wear agent (or an extreme-pressure agent). Examples thereof include sulfur-based, phosphorus-based, and sulfur-phosphorus-based extreme pressure agents, and specifically, phosphorous esters, thiophosphorous esters, dithiophosphorous esters, trithiophosphorous esters, phosphate esters, thiophosphate esters, dithiophosphate esters, trithiophosphate esters, amine salts thereof, metal salts thereof, derivatives thereof, dithiocarbamate, zinc dithiocarbamate, disulfides, polysulfides, sulfurized olefins, and sulfurized oils. Among them, addition of a sulfur-based extreme-pressure agent, especially sulfurized oil is preferable.

Examples of corrosion inhibitors include benzotriazole compounds, tolyltriazole compounds, thiadiazole compounds, and imidazole compounds.

Examples of anti-rust agents include petroleum sulfonate, alkylbenzenesulfonate, dinonylnaphthalenesulfonate, alkenylsuccinate esters, and polyol esters.

Examples of demulsifiers include polyoxyalkylene glycol-based nonionic surfactants such as polyoxyethylene alkyl ether, polyoxyethylene alkylphenyl ether, and polyoxyethylene alkylnaphthyl ether.

Examples of metal deactivators include imidazoline, pyrimidine derivatives, alkylthiadiazole, mercaptobenzothiazole, benzotriazole or derivatives thereof, 1,3,4-thiadiazole polysulfide, 1,3,4-thiadiazolyl-2,5-bisdialkyldithiocarbamate, 2-(alkyldithio)benzimidazole, and β-(o-carboxybenzylthio)propionitrile.

Examples of defoaming agents include silicone oil having kinematic viscosity at 25° C. of 1000 to 100,000 mm²/s, alkenylsuccinimide derivatives, esters of polyhydroxy aliphatic alcohol and long-chain fatty acid, methyl salicylate, and o-hydroxybenzylalcohol.

When any of these additives are incorporated in the lubricating oil composition of the present invention, each of the contents thereof is preferably 0.01 to 10 mass % on the basis of the total mass of the lubricating oil composition.

<Lubricating Oil Composition>

The kinematic viscosity of the lubricating oil composition of the present invention at 100° C. is preferably 4.0 to 12 mm²/s, more preferably no more than 9.0 mm²/s, further preferably no more than 8.0 mm²/s, especially preferably no more than 7.0 mm²/s, and most preferably no more than 6.8 mm²/s; and more preferably no less than 4.5 mm²/s, further preferably no less than 5.0 mm²/s, especially preferably no less than 5.5 mm²/s, and most preferably no less than 6.0 mm²/s. The kinematic viscosity at 100° C. here is the kinematic viscosity at 100° C. specified in ASTM D-445. The kinematic viscosity of the lubricating oil composition at 100° C. under 4.0 mm²/s may lead to insufficient lubricity. The kinematic viscosity thereof beyond 12 mm²/s may lead to insufficient low-temperature viscosity and fuel efficiency

The kinematic viscosity of the lubricating oil composition of the present invention at 40° C. is preferably 4.0 to 50 mm²/s, more preferably no more than 40 mm²/s, further preferably no more than 35 mm²/s, especially preferably no more than 32 mm²/s, and most preferably no more than 30 mm²/s; and more preferably no less than 15 mm²/s, further preferably no less than 18 mm²/s, further more preferably no less than 20 mm²/s, especially preferably no less than 22 mm²/s, and most preferably no less than 25 mm²/s. The kinematic viscosity at 40° C. here is the kinematic viscosity at 40° C. specified in ASTM D-445. The kinematic viscosity of the lubricating oil composition at 40° C. under 4 mm²/s may lead to insufficient lubricity. The kinematic viscosity thereof beyond 50 mm²/s may lead to insufficient low-temperature viscosity and fuel efficiency.

The viscosity index of the lubricating oil composition of the present invention is preferably 140 to 400, more preferably no less than 150, further preferably no less than 160, and especially preferably no less than 165. In a case where the viscosity index of the lubricating oil composition is under 140, it might be difficult to improve the fuel efficiency while keeping the HTHS viscosity at 150° C., and further, to reduce the low-temperature viscosity (for example, at −35° C. that is measurement temperature of the CCS viscosity specified in the SAE viscosity grade OW-X, known as viscosity grades of fuel-economy oil). When the viscosity index of the lubricating oil composition is beyond 400, the evaporation loss might increase, and troubles due to insufficient solubility of additives and compatibility with seal materials might occur.

The HTHS viscosity of the lubricating oil composition of the present invention at 100° C. is preferably no more than 5.5 mPa·s, more preferably no more than 5.0 mPa·s, further preferably no more than 4.9 mP·a, and especially preferably no more than 4.8 mPa; and preferably no less than 3.0 mPa·s, more preferably no less than 3.5 mPa·s, further preferably no less than 4.0 mPa, and especially preferably no less than 4.5 mPa·s. In this application, the HTHS viscosity at 100° C. is high temperature high shear viscosity at 100° C., specified in ASTM D4683. The HTHS viscosity at 100° C. under 3.0 mPa·s may lead to insufficient lubricity. The HTHS viscosity at 100° C. beyond 5.5 mPa·s may lead to insufficient low-temperature viscosity and fuel efficiency.

The HTHS viscosity of the lubricating oil composition of the present invention at 150° C. is preferably no more than 4.0 mPa·s, more preferably no more than 3.0 mPa·s, further preferably no more than 2.7 mPa·s, and especially preferably no more than 2.5 mPa·s; and preferably no less than 2.0 mPa·s, more preferably no less than 2.1 mPa·s, further preferably no less than 2.2 mPa·s, and especially preferably no less than 2.25 mPa·s. In this application, the HTHS viscosity at 150° C. is high temperature high shear viscosity at 150° C., specified in ASTM D4683. The HTHS viscosity at 150° C. under 2.0 mPa·s may lead to insufficient lubricity. The HTHS viscosity at 150° C. beyond 4.0 mPa·s may lead to insufficient fuel efficiency.

The evaporation loss of the lubricating oil composition according to the first embodiment of the present invention is, as NOACK evaporation loss at 250° C., preferably no more than 20 mass %, further preferably no more than 16 mass %, and especially preferably no more than 15 mass %. When the NOACK evaporation loss is beyond 20 mass %, the evaporation loss of the lubricating oil is large, which causes viscosity increase and the like, and is not preferable. The NOACK evaporation loss here is evaporation loss of the lubricating oil measured conforming to ASTM D 5800. The lower limit of the NOACK evaporation loss of the lubricating oil composition at 250° C. is not restricted, but normally no less than 5 mass %.

The NOACK evaporation loss of the lubricating oil composition according to the second embodiment of the present invention at 250° C. is preferably no less than 15 mass %, further preferably no less than 20 mass %, and further preferably no less than 25 mass %. The upper limit of the NOACK evaporation loss thereof is not restricted, but typically no more than 50 mass %, and preferably no more than 40 mass %.

The lubricating oil composition according to the first embodiment of the present invention has improved durability, fuel efficiency, and evaporation prevention, and can keep sufficiently low kinematic viscosity at 40° C. and 100° C. at the initial stage of use thereof as well as for an extended period of use thereof, while maintaining the HTHS viscosity at 150° C., and can sufficiently suppress viscosity decrease after being sheared. The lubricating oil composition according to the second embodiment of the present invention has improved durability and fuel efficiency, and can keep sufficiently low kinematic viscosity at 40° C. and 100° C. at the initial stage of use thereof as well as for an extended period of use thereof, while maintaining the HTHS viscosity at 150° C., and can sufficiently suppress viscosity decrease after being sheared. The lubricating oil composition of the present invention having the above described excellent characteristics can be preferably used as fuel-economy engine oils such as fuel-economy gasoline engine oil and fuel-economy diesel engine oil.

EXAMPLES

Hereinafter the present invention will be more specifically described based on the examples and comparative examples. It is noted that the present invention is not limited to these examples.

Examples 1 to 8 and Comparative Examples 1 to 9

Using the following base oils and additives, each of the lubricating oil compositions according to the first embodiment of the present invention (Examples 1 to 4) and lubricating oil compositions for comparison (Comparative Examples 1 to 5) was prepared; and in addition, each of the lubricating oil compositions according to the second embodiment of the present invention (Examples 5 to 8) and lubricating oil compositions for comparison (Comparative Examples 6 to 9) was also prepared. Table 2 shows the properties of the base oils 0-1 to 0-4. Table 3 shows the viscosity increase effects A to C, which were measured for a mixture of YUBASE™ 4 manufactured by SK Lubricants Co., and each viscosity index improver added thereto by 6.0 mass % on the basis of the total mass of the mixture, and their ratios A/B and C/B. Tables 4 and 5 show the composition and properties of the lubricating oil composition of each of Examples 1 to 8 and Comparative Examples 1 to 9 (kinematic viscosity at 40° C. or 100° C., viscosity index, and HTHS viscosity at 100° C. or 150° C.). In Tables, “inmass %” represents mass % on the basis of mass of the total base oil, “mass %” represents mass % on the basis of the total mass of the composition, and “mass ppm” represents mass ppm on the basis of the total mass of the composition.

(Base Oil)

Base oil O-1 (base oil 1): hydrocracked mineral oil (YUBASE™ 4 manufactured by SK Lubricants Co.)

Base oil O-2 (base oil 2): mineral oil obtained by hydrocracking/hydroisomerizing n-paraffin-containing oil

Base oil O-3 (base oil 3): hydrocracked mineral oil

Base oil O-4 (base oil 4): wax isomerized mineral oil obtained by hydrocracking/hydroisomerizing n-paraffin-containing oil

(Viscosity Index Improver)

PMA-1: a non-dispersant polymethacrylate viscosity index improver (a copolymer obtained by copolymerizing a methacrylate of the general formula (2) wherein R⁴ was an alkyl group having a carbon number of 6 or less, and a methacrylate of the general formula (3) wherein R⁶ was an alkyl group having a carbon number of 7 or more. Mw=1.5×10⁵, Mn=6.0×10⁴, Mw/Mn=2.5, PSSI=0.1, and Mw/PSSI=1.5×10⁶)

PMA-2: a non-dispersant comb-shaped polymethacrylate viscosity index improver (a copolymer obtained by copolymerizing a methacrylate of the general formula (2) wherein R⁴ was an alkyl group having a carbon number of 6 or less, and a methacrylate of the general formula (3) wherein R⁶ was an alkyl group having a carbon number of 7 or more. Mw=2.0×10⁵, Mn=8.0×10⁵, Mw/Mn=2.6, PSSI=0.1, and Mw/PSSI=2.0×10⁶)

PMA-3: a non-dispersant polymethacrylate viscosity index improver (a copolymer obtained by copolymerizing a methacrylate of the general formula (2) wherein R⁴ was an alkyl group having a carbon number of 6 or less, and a methacrylate of the general formula (3) wherein R⁶ was an alkyl group having a carbon number of 7 or more. Mw=4.0×10⁵, Mn=1.3×10⁵, Mw/Mn=3.1, PSSI=4, and Mw/PSSI=1×10⁵)

PMA-4: a non-dispersant polymethacrylate viscosity index improver (a copolymer obtained by copolymerizing a methacrylate of the general formula (2) wherein R⁴ was an alkyl group having a carbon number of 6 or less, and a methacrylate of the general formula (3) wherein R⁶ was an alkyl group having a carbon number of 7 or more. Mw=4.0×10⁵, Mn=1.3×10⁵, Mw/Mn=3.1, PSSI=30, Mw/PSSI=1.4×10⁴)

PMA-5: a dispersant polymethacrylate viscosity index improver (a copolymer obtained by copolymerizing a methacrylate of the general formula (2) wherein R⁴ was an alkyl group having a carbon number of 6 or less, a methacrylate of the general formula (3) wherein R⁶ was an alkyl group having a carbon number of 7 or more, and (dimethylamino)ethyl methacrylate. Mw=3.0×10⁵, Mn=7.5×10⁴, Mw/Mn=4.0, PSSI=40, and Mw/PSSI=7.3×10³)

PMA-6: a non-dispersant polymethacrylate viscosity index improver (a copolymer obtained by copolymerizing a methacrylate of the general formula (2) wherein R⁴ was an alkyl group having a carbon number of 6 or less, and a methacrylate of the general formula (3) wherein R⁶ was an alkyl group having a carbon number of 7 or more. Mw=3.5×10⁵, PSSI=30, Mw/PSSI=1.2×10⁴)

(Friction Modifier)

B-1: MoDTC (alkyl groups were combination of an alkyl group of chain length C8 and an alkyl group of chain length C13, Mo content: 10 mass %, sulfur content: 11 mass %)

(Other Additives)

C-1: performance additive package A (an additive package including succinimide dispersant, ZnDTP, an antioxidant, an anti-wear agent, a pour point depressant, and a defoaming agent)

D-1: metallic detergent A (overbased calcium salicylate, base number: 170 mgKOH/g, Ca content: 6.3 mass %)

D-2: metallic detergent B (calcium borate-overbased calcium salicylate, base number: 190 mgKOH/g, Ca content: 6.8 mass %, B content: 2.7 mass %)

TABLE 2 O-1 O-2 O-3 O-4 Base Base Base Base Oil 1 Oil 2 Oil 3 Oil 4 Density (15° C.) g/cm³ 0.835 0.820 0.832 0.808 Kinematic Viscosity mm²/s 20.0 15.8 13.5 9.3 (40° C.) (100 C.) mm²/s 4.29 3.85 3.3 2.7 Viscosity Index 123 141 112 127 Pour Point ° C. −17.5 −22.5 −22.5 −32.5 NOACK Evaporation mass % 14 13 34 39 Loss (250° C., 1 h) Aniline Point ° C. 116 119 109 112 Iodine Number 0.05 0.06 5.38 — S Content mass ppm <1 <1 <1 <0.5 N Content mass ppm <3 <3 <3 — Analysis by n-d-M % C_(P) 80.7 93.3 72.6 90.2 Method % C_(N) 19.3 6.7 27.4 9.8 % C_(A) 0 0 0 0 Chromatographic Saturated 99.7 99.6 99.6 99.6 Analysis mass % Cont. Aromatic 0.2 0.2 0.3 0.2 Cont. Resin 0.1 0.1 0.1 0.2 Cont. Recovery 100 99.9 100 100 Rate Paraffin Cont. mass % 53.8 87.1 50.7 — on Basis of Saturated Cont. Naphthene Cont. mass % 46.2 12.9 49.3 — on Basis of Saturated Cont. Producing Method Hydro- Wax Hydro- Wax crack- Isom- crack- Isom- ing erizing ing erizing

TABLE 3 Viscosity Index Improver PMA-1 PMA-2 PMA-3 PMA-4 PMA-5 PMA-6 YUBASE4 mass % Balance Balance Balance Balance Balance Balance Additive Amount of VIP 6.0 6.0 6.0 6.0 6.0 6.0 Kinematic Viscosity Increase Effect A (100° C.) mm²/s 1.06 0.94 1.16 0.85 3.94 1.50 Kinematic Viscosity Increase Effect C (150° C.) mm²/s 0.57 0.48 0.70 0.49 1.97 0.81 HTHS Viscosity Increase Effect B (150° C.) mPa · s 0.47 0.45 0.46 0.29 0.92 0.42 A/B 2.26 2.09 2.53 2.90 4.30 3.54 C/B 1.23 1.07 1.52 1.67 2.15 1.92

(Evaluation of Lubricating Oil Composition)

The kinematic viscosity at 40° C. or 100° C., the viscosity index, and the HTHS viscosity at 100° C. or 150° C. were measured for each of the lubricating oil compositions of Examples 1 to 8 and Comparative Examples 1 to 9. Engine motoring friction test was also carried out for each of the lubricating oil compositions of Examples 1 to 8 and Comparative Examples 1 to 2, 5, 6 to 7 and 9. Measurement methods were as follows:

(1) kinematic viscosity: measured conforming to ASTM D-445. (2) viscosity index: measured conforming to JIS K 2283-1993. (3) HTHS viscosity: measured conforming to ASTM D-4683 (4) engine motoring friction test: friction torque was measured at 60 to 95° C. in oil temperature at 750 to 3,000 rpm in engine speed with a roller rocker type 1,800 cc inline four-cylinder engine. Table 4 shows the motoring friction average reduction rates calculated using Comparative Example 1 as the reference oil. Table 5 shows the motoring friction average reduction rates calculated using Comparative Example 5 as the reference oil.

As understood from Table 3, for the reference base oil YUBASE™ 4, the viscosity index improvers PMA-1 and PMA-2 satisfied the requirement concerning the ratio of the viscosity increase effects, that is, “A/B<2.4 and C/B<1.4”, and PMA-3 to PMA-5 did not satisfy the requirement.

TABLE 4 Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 1 Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Base Oil O-1 inmass % 50 50 100 95 100 100 100 100 100 Base Oil O-2 inmass % 50 50 Base Oil O-3 inmass % 5 Base Oil Kinematic Viscosity  40° C. mm²/s 17.7 17.7 20.0 19.0 20.0 20.0 20.0 20.0 20.0 100° C. mm²/s 4.06 4.06 4.29 4.19 4.29 4.29 4.29 4.29 4.29 Viscosity Index 131 131 123 126 123 123 123 123 123 Viscosity index improver PMA-1 mass % 5.4 3.7 3.9 3.7 PMA-2 mass % 5.8 PMA-3 mass % 4.9 PMA-4 mass % 7.4 PMA-5 mass % 2.7 PMA-6 mass % 5.5 B-1 Friction Modifier A mass % 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 — C-1 Performance Additive Package A mass % 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 D-1 Metallic Detergent A mass % 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 D-2 Metallic detergent B mass % 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Lubricating Oil Composition Kinematic Viscosity  40° C. mm²/s 26.7 26.2 29.1 28.8 27.9 29.9 33.4 30.6 29.0 100° C. mm²/s 6.19 6.13 6.26 6.25 6.55 6.74 7.23 6.98 6.25 Viscosity Index 197 198 173 176 202 193 186 199 174 HTHS Viscosity 100° C. mPa · s 4.64 4.50 4.78 4.76 4.65 4.56 4.82 4.67 4.78 150° C. mPa · s 2.28 2.28 2.28 2.28 2.28 2.28 2.28 2.28 2.28 NOACK Evaporation Loss 250° C. mass % 14 14 14 14 14 14 14 14 14 Engine Motoring Friction % 0.5 0.8 0.3 0.4 reference −0.5 — — −1.0 Reduction Rate

TABLE 5 Comp. Comp. Comp. Ex. 5 Ex. 6 Ex. 7 Ex. 8 Comp. Ex. 6 Ex. 7 Ex. 8 Ex. 9 Base Oil O-1 inmass % 80 50 50 100 100 100 100 Base Oil O-2 inmass % 35 Base Oil O-3 inmass % 20 50 50 Base Oil O-4 inmass % 65 Base Oil Kinematic Viscosity  40° C. mm²/s 19.1 17.7 17.7 11.1 20.0 20.0 20.0 20.0 100° C. mm²/s 4.20 4.06 4.06 3.04 4.29 4.29 4.29 4.29 Viscosity Index 126 131 131 137 123 123 123 123 Viscosity Index Improver PMA-1 mass % 2.2 3.4 6.5 1.4 PMA-2 mass % 3.8 PMA-3 mass % 1.9 PMA-4 mass % 3.0 PMA-5 mass % 1.0 B-1 Friction Modifier A mass % 0.8 0.8 0.8 0.8 0.8 0.8 0.8 — C-1 Performance Additive mass % 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 Package A D-1 Metallic Detergent A mass % 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 D-2 Metallic Detergent B mass % 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Lubricating Oil Composition Kinematic Viscosity  40° C. mm²/s 25.4 23.8 23.3 19.6 26.1 26.7 27.1 26.5 100° C. mm²/s 5.49 5.42 5.44 5.27 5.61 5.65 5.73 5.53 Viscosity Index 164 178 188 230 164 164 159 153 HTHS Viscosity 100° C. mPa · s 4.24 4.13 4.07 3.80 4.32 4.30 4.30 6.51 150° C. mPa · s 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 NOACK Evaporation Loss 250° C. mass % 18 24 24 29 14 14 14 14 Engine Motoring Friction % 0.6 1.0 1.3 1.6 reference −0.4 — −0.8 Reduction Rate

As shown in Table 4, the lubricating oil compositions of Examples 1 to 4 according to the first embodiment of the present invention had lower kinematic viscosity and showed better fuel efficiency compared to the engine oil compositions of Comparative Examples 1 to 4, which had the approximately same HTHS viscosity at 150° C. as Examples 1 to 4, but did not contain the component (A). The lubricating oil composition of Comparative Example 5, which did not contain the component (B), showed significantly poor motoring friction reduction rate.

As shown in Table 5, the lubricating oil compositions of Examples 5 to 8 according to the second embodiment of the present invention had lower kinematic viscosity and showed better fuel efficiency compared to the engine oil compositions of Comparative Examples 6 to 8, which had the approximately same HTHS viscosity at 150° C. as Examples 5 to 8, but did not contain the component (A). The lubricating oil composition of Comparative Example 9, which did not contain the component (B), showed significantly poor motoring friction reduction rate.

INDUSTRIAL APPLICABILITY

The lubricating oil composition of the present invention can be preferably used as a lubricating oil composition for internal combustion engines such as gasoline engines, diesel engines and gas engines for two and four wheeled vehicles, electric power generation, cogeneration, and so on. Other than the above, the lubricating oil composition of the present invention can be preferably used as transmission fluid, ATF, CVTF, MTF, hydraulic fluid, and so on. Further, the lubricating oil composition of the present invention is useful as a lubricating oil composition for various engines that use fuels containing no more than 50 mass ppm of sulfur, and various engines for ships, and outboard motors. 

1. A lubricating oil composition comprising: a lubricant base oil comprising one or more base oil, the lubricant base oil having a kinematic viscosity at 100° C. of 1.0 to 10 mm²/s and % C_(P) of no less than 70; (A) 0.1 to 30 mass % of a poly(meth)acrylate viscosity index improver having a PSSI of no more than 5, a weight average molecular weight of 10,000 to 500,000, a ratio A/B of less than 2.4 and a ratio C/B of less than 1.4, the ratio A/B being a ratio of a viscosity increase effect A on kinematic viscosity at 100° C. represented by the following formula (1) to a viscosity increase effect B on HTHS viscosity at 150° C. represented by the following formula (2), and the ratio C/B being a ratio of a viscosity increase effect C on kinematic viscosity at 150° C. represented by the following formula (3) to the viscosity increase effect B on HTHS viscosity at 150° C. represented by the following formula (2); and (B) 0.01 to 2.0 mass % of a friction modifier, A=X−X ₀  (1) wherein in the formula (1), A represents the viscosity increase effect of the viscosity index improver on kinematic viscosity at 100° C.; X represents kinematic viscosity at 100° C. (unit: mm²/s) of a mixture consisting of a reference base oil and 6 mass % of the viscosity index improver, the reference base oil being a hydrocracked base oil YUBASE™ 4 manufactured by SK Lubricants Co., Ltd.; and X₀ represents kinematic viscosity at 100° C. (unit: mm²/s) of the reference base oil; B=Y−Y ₀  (2) wherein in the formula (2), B represents the viscosity increase effect of the viscosity index improver on HTHS viscosity at 150° C.; Y represents HTHS viscosity at 150° C. (unit: mPa·s) of the mixture consisting of the reference base oil and 6 mass % of the viscosity index improver; and Y₀ represents HTHS viscosity at 150° C. (unit: mPa·s) of the reference base oil; and C=Z−Z ₀  (3) wherein in the formula (3), C represents the viscosity increase effect of the viscosity index improver on kinematic viscosity at 150° C.; Z represents kinematic viscosity at 150° C. (unit: mm²/s) of the mixture consisting of the reference base oil and 6 mass % of the viscosity index improver; and Z₀ represents kinematic viscosity at 150° C. (unit: mm²/s) of the reference base oil.
 2. The lubricating oil composition according to claim 1, wherein the lubricant base oil has a NOACK evaporation loss at 250° C. of less than 15 mass %.
 3. The lubricating oil composition according to claim 2, wherein the (A) viscosity index improver has a ratio (Mw/PSSI) of the weight average molecular weight (Mw) to the PSSI of no less than 1×10⁴.
 4. The lubricating oil composition according to claim 2, wherein the (B) friction modifier is a molybdenum-based friction modifier.
 5. The lubricating oil composition according to claim 2, wherein the lubricating oil composition has a NOACK evaporation loss at 250° C. of no more than 15 mass %.
 6. The lubricating oil composition according to claim 1, wherein the lubricant base oil has a NOACK evaporation loss at 250° C. of no less than 15 mass %; and the poly(meth)acrylate viscosity index improver has a weight average molecular weight of 10,000 to 400,000.
 7. The lubricating oil composition according to claim 6, wherein the (A) viscosity index improver has a ratio (Mw/PSSI) of the weight average molecular weight (Mw) to the PSSI of no less than 1×10⁴.
 8. The lubricating oil composition according to claim 6, wherein the (B) friction modifier is a molybdenum-based friction modifier.
 9. The lubricating oil composition according to claim 6, wherein the lubricating oil composition has a NOACK evaporation loss at 250° C. of no less than 15 mass %.
 10. The lubricating oil composition according to claim 6, wherein the lubricant base oil is a wax isomerized base oil having a kinematic viscosity at 100° C. of 2.0 to 4.5 mm²/s, % C_(P) of no less than 85, and a NOACK evaporation loss at 250° C. of no less than 15 mass %. 